Energy saving materials are actively developed and applied in all industry area due to the situation of energy source which reached the limit. In general, a near infrared ray absorbing material (pigment) is used in order to prevent electronic devices from malfunctioning by interrupting the near infrared ray generated in the electronic devices. However, when attaching a film containing the material to a window, it can absorb part of sunlight, thereby reducing heat exchange between inside and outside, and thus prevent excessive rise and drop in room temperature. Accordingly, such a near infrared absorbing material can be used as a representative energy saving material, and is actively applied in construction and automobile industry. The near infrared absorbing material used in the film to be attached to a window of an automobile or a construction should have superior performance in absorbing the near infrared ray simultaneously with minimum absorbance at visible light region. That is because when a near infrared absorbing material having high absorbance at visible light region is used in the film to be attached to a window of an automobile or a construction, the automobile driver's view may become blurred or the house may command a bad view, and additional energy for internal light collection may be demanded or its color may be not natural.
Representative near infrared absorbing materials (pigments) include a diimmonium-based material (compound), a phthalocyanine-based material, a cyanine-based material and a metal dithiolene-based material, etc. However, there are little material that can be applied to a window and an electronic devices, etc. of a real automobile or a construction due to their physical or chemical properties in addition to the optical properties. The phthalocyanine-based material, which is the representative near infrared absorbing material, is actively applied for electronic devices including display due to its chemical stability. But the phthalocyanine-based material has a drawback in that it cannot be applied alone for a film, etc. for an automobile window because it has high absorbance in visible light region. The cyanine-based material has a drawback in that it has low chemical stability, and has narrow widths of the near infrared region which it can absorb alone, thus for actual application, the absorbing regions should be widened by mixing several materials. Further, the metal dithiolene-based material absorbs near infrared ray highly, but has a drawback in that it requires dispersion equipments, etc. for applying to a film due to its low solubility and is difficult to apply for a use requiring high transmittance.
However, the diimmonium-based material has characteristics that it absorbs near infrared ray at wide region, and hardly absorbs at the whole region of visible light, and thus even only single material can absorb the whole region of near infrared ray. Further, the diimmonium-based material has been often used conventionally in the window market of an automobile and a house since it has chemical stability and superior solubility to various organic solvent, and thus is easy to apply to a process. The main construction of the diimmonium-based near infrared absorbing material is comprised of cations having diimmonium structure and various anions. Representative anions include a halogen ion such as fluorine, chlorine and bromine, an inorganic-atom-containing ion such as hexafluoroantimonate ion, hyperchlorate ion and tetrafluoroborate ion, and an organic carbonate ion such as acetate ion and lactate ion, and these were initially used. Recently, an anion such as bis(trifluoromethanesulfonyl)imide ion and bis(pentafluoroethanesulfonyl) imide ion is much used. However, the diimmonium-based compounds which use the halogen ions as anions are difficult to apply to the process which uses them as a near infrared absorbing material, due to solubility problem. The organic carbonate ion is difficult to use actually since its solubility to water is high, thereby rendering yield control difficult in producing the diimmonium-based compound, and causing weatherability problem to a product. Further, anion material which is recently used widely, for example, the fluoroantimonic acid ions can be used as a near infrared absorbing material by their proper properties, but is not proper to use in a place closely related to human residential environment such as a house and an automobile since toxic material such as hydrofluoric acid can be emitted due to the fluorine atom present in the material, and the durability of the product (film, etc.) can be reduced by hydrofluoric acid.